I've never used Metcalf and Eddy myself.
I used two books for the water/wastewater sections.
"Unit Operations and Processes in Environmental Engineering", by Reynolds and Richards
"Environmental Biotechnology: Principles and Applications", by Rittman and McCarty
Unit Ops was great for the physical process stuff, talks about typical processes and variations on them. It gives typical operations conditions, common operational problems, etc. It's weak on activated sludge and kinetics though.
The Biotech book is really dense an technical but it's a great book. It goes way into depth on microbial kinetics, mass balances, solving for substrate and biomass, etc.
I answered every Wtr/WW problem on the test using those 2 books.